1. Field of the Invention
The present invention relates to a semiconductor device, a liquid crystal display device, an EL display device, and methods for manufacturing these devices.
In this specification, a semiconductor device means all types of devices that can function by utilizing semiconductor characteristics, and a transistor, a semiconductor circuit, a memory device, an imaging device, a display device, an electro-optical device, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
Display devices, such as active matrix liquid crystal display devices and electroluminescent (EL) display devices, which include transistors have been in practical use. Transistors are widely used for electronic devices such as integrated circuits (ICs) as well as for display devices.
In recent years, demands for display devices with larger screen size, higher definition, and higher aperture ratio are increasing. In addition, display devices are required to have high reliability and be produced at low cost.
In liquid crystal display devices and EL display devices, transistors are used as switching elements, driving transistors, and the like. Therefore, it is important to improve the reliability of the transistor for the display device to have high reliability. In addition, it is effective to simplify the manufacturing process of the transistor in order to simplify the manufacturing process of the display device for the purpose of reducing production cost.
Suppressing deterioration of a semiconductor layer is an effective way to improve the reliability of a transistor. Deterioration of the semiconductor layer is caused by damage due to an etching step in a photolithography step or by diffusion of an impurity element from another layer to the semiconductor layer. Therefore, in order to suppress deterioration of the semiconductor layer, it is effective to provide a layer for reducing damage to the semiconductor layer due to a manufacturing step such as a photolithography step or to provide a blocking layer for preventing diffusion of an impurity element from another layer to the semiconductor layer.
Meanwhile, reduction in the number of photolithography steps or simplification of the photolithography step is effective for simplification of a manufacturing process of a transistor. For example, when one photolithography step is added, the following steps are further needed: resist application, prebaking, light exposure, development, post-baking, and the like and, moreover, steps before and after the aforementioned steps, such as film formation, etching, resist removal, cleaning, and drying. The number of steps is significantly increased only by adding one photolithography step in the manufacturing process.
As an example, in a manufacturing process of a bottom-gate transistor which is used as a switching element in a large-sized liquid crystal display device, the transistor is generally manufactured through photolithography steps using at least five photomasks. Specifically, in the case of a transistor for a pixel in the liquid crystal display device, five photolithography steps are needed: a step of forming a gate electrode (including a wiring formed in the same layer), a step of forming an island-shaped semiconductor layer, a step of forming a source electrode and a drain electrode (including a wiring formed in the same layer), a step of forming an opening (contact hole) (the step includes removal of an insulating layer or the like in a region other than the opening), and a step of forming a pixel electrode (including a wiring or the like formed in the same layer). Accordingly, omitting one photomask or simplifying the photolithography step is highly effective in shortening the manufacturing process of the transistor.
As another example, in a manufacturing process of a bottom-gate transistor which is used as a switching element in a large-sized EL display device, the transistor is generally manufactured through photolithography steps using at least six photomasks. Specifically, in the case of a transistor for a pixel in the EL display device, six photolithography steps are needed: a step of forming a gate electrode (including a wiring formed in the same layer), a step of forming an island-shaped semiconductor layer, a step of forming a source electrode and a drain electrode (including a wiring formed in the same layer), a step of forming an opening (contact hole) (the step includes removal of an insulating layer or the like in a region other than the opening), a step of forming one electrode of an EL element (including a wiring or the like formed in the same layer), and a step of forming a partition layer used for separately coloring EL layers with corresponding colors. Accordingly, omitting one photomask or simplifying the photolithography step is highly effective in shortening the manufacturing process of the transistor.
Therefore, many techniques for reducing the number of photolithography steps or simplifying the photolithography step in a manufacturing process of a transistor have been developed. Examples of techniques for simplifying the photolithography step include techniques such as backside light exposure, resist reflow, and a lift-off method. A technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known as a technique for simplifying the photolithography step in a manufacturing process of a transistor. A technique for reducing the number of manufacturing steps by using a multi-tone mask is disclosed in Patent Document 1, for example.